gitlab-ci: image build should fail on failed commands

Ensure that no command in the script fails silently. If any command fails, the
image is likely broken.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

.gitlab-ci/debian-install.sh

 #!/bin/bash
 
-set -o xtrace
+set -o xtrace -o errexit
 
 echo 'deb http://deb.debian.org/debian buster-backports main' >> /etc/apt/sources.list
 apt-get update